1. Field of the Invention
This invenstion relates to a compound bow and more particularly to a pulley mechanism for controlling the speed and draw capabilities of such bows.
2. Description of Prior Art
The bow and arrow is a fundamental tool of civilization. From the earliest times, hunters have been bending limbs and using the resilient force of the bent wood to propel arrows in seach of food and for self-protection. Isaac Newtown provided us with the mathematical relationships showing that the distance an arrow will travel without dropping is a function of the speed of the arrow as it leaves the bow. Wars have been fought with bows and arrows and, as one would expect, the search continues for bows having great accuracy and thus high arrow speed.
Arrow speed is determined by how fast a pulled drawstring returns to the neutral position. This speed, in turn, is traditionally determined by the hysterisis (memory) effect of the bent bow limb. Since the speed with which a bent bow returns to the neutral position is directly related to the force required to bend the bow limb, it follows that as higher arrow speeds are desired larger draw forces are required to bend the limb. Accordingly, since there is a practical limit to how much force a person can use to pull the drawstring, there is an upper limit on arrow speed.
A major solution to this problem was the advent of the compound bow, where a pulley system is used to allow the drawstring to be pulled back a large distance while only requiring the limb of the bow to move a much smaller distance.
Typically, a compound bow has two pulleys (usually in the shape of cams) one we will call the drawstring pulley and the other, for convenience, we will call the power or tension pulley. These two pulleys are connected together to form a set and the end of each limb of the bow is fitted with such a set. The tension pulley has a tension cable attached permanently to it is the circumferential rim, the other end of the cable being attached to the other limb end. The tension cables from the two tension pulleys (one at each limb end) thus cross each other at the center of the bow. The drawstring pulleys each terminate one end of the drawstring on their respective circumferential rims, such that as the drawstring is pulled back the two drawstring pulleys rotate. This rotation causes the two tension pulleys to also rotate thereby winding a portion of the tension cables onto each tension pulley. Since the far end of each tension cable is fixed to an end of the bow limb, the limbs bend as the drawstring is pulled into the drawn position.
When the drawstring is fully drawn, the cam nature of the tension pulleys "break-over" thereby allowing the drawstring to be held with much less force than is actually required to pull the string back.
When the drawstring is released, the limbs return to their normal condition causing the tension pulleys to unwind, thereby pulling the drawstring forward.
Compound bows, which partially solve the bow bending dilemma, have become very popular in recent years. However, even the best compound bow has limitations in the speed of forward movement of a released drawstring. This speed is controlled by the mechanism used to return the drawstring to its normal position. Typically, with a compound bow, the tension cable moves four inches representing a four inch bending of the limb. Thus, the tension cable must move four inches while the drawstring moves about six inches from the normal position to the fully drawn position.
Because the draw string is attached to a cam which is off center to give leverage and because this cam is fixed to the tension cam which controls the tension cable, this relationship is fixed. Or to say it another way, becasue of the relative circumferential areas of the two cams, there can only be an approximately 3:1 difference in string length between the drawstring and the tension cable. If the relative sizes of two cams were to be changed then the drawstring cam would become so large that it would unbalance the bow because of the mass at the ends of the bow limbs. Speed might possibly be increased, but only at the price of increased arrow wobble due to the vibration of the bow as the large mass of the cams move from the drawn position to the normal position.
Another problem stemming from the use of compound bows is the use of the cam itself. This problem comes from the "break-over" motion of the cam as it goes "over-center" near the end of the draw. Upon release of the drawstring the over center action of the cam imparts a vibration to the arrow which affects the arrow's flight path as well as its ultimate accuracy.
A still further problem with compound bows is that as the force increases, the bow arms are forced to move further and thus have a tendency to fatique and break.
Thus, it is desirable to design a compound bow gear system which allows for high arrow speed without introducing quiver to the arrow as it leaves the drawstring.
It is further desirable to provide such a compound gear system which allows a stronger bow limb to be used without introducing additional drawstring pulling forces and without requiring large bending movements of the bow arms.
It is further desirable to design a mechanism for a compound bow which eliminates the "break-over" problem and which also does not introduce bow quiver by the movement of a large mass at the bow limb ends.
It is a further desire to design such a mechanism which will impart a faster arrow speed upon release of the drawstring while allowing the bow arm limited movement.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more pertinent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention is a different manner of modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.